1. Field of the Invention
The present invention relates to Lithium Metal batteries. In particular, it is related to lithium metal batteries containing a polyimide-based electrolyte.
2. Background of the Invention
During the last ten years, lithium batteries of the primary and rechargeable type have been the object of a considerable number of research and development works. The intent was to develop a battery which is safe, inexpensive, having a large energetic content and good electrochemical performance. In this context, a plurality of a battery designs were developed to meet different applications, such as microelectronics, telecommunications, portable computers and electrical vehicles, to name only a few.
Electrochemical batteries or generators, whether rechargeable or not, are all made of an anode which can consist of a metal such as lithium and alloys thereof, or an insertion compound which is reversible towards lithium, such as carbon, a cathode which consists of an insertion compound which is reversible towards lithium such as transitional metal oxide, a mechanical separator and an electrolytic component placed in between the electrodes. The term electrolytic component means any material placed inside the generator and which is used as ionic transport except electrode materials in which the ions Li+ may be displaced. During the discharge or charge of the generator, the electrolytic component ensures the transport of ionic species through the entire generator from one electrode to the other and even inside the composite electrodes. In lithium batteries, the electrolytic component is generally in the form of a liquid which is called liquid electrolyte or a dry or gel polymer matrix which may also act as mechanical separator.
When the electrolytic component is in liquid form, it consists of an alkali metal salt which is dissolved in an aprotic solvent. In the case of a lithium generator, the more common salts are LIPF6, LIBF4 and LiN(SO2CF3)2 and the polar aprotic solvents may be selected from propylene carbonate, ethylene carbonate, Y-butyrolactone and 1,3-dioxolane or their analogs to name only a few. At the level of the separator, the liquid electrolyte is generally impregnated in a porous polymer matrix which is inert towards the aprotic solvent used, or in a fiberglass paper. The use of a liquid electrolyte which is impregnated in an inert polymer matrix enables to preserve a sufficient ionic mobility to reach a level of conductivity of the order of 10−3 S/cm at 25° C. At the level of the composite electrodes, when the latter are made of an insertion material which is bound by a polymer matrix which is inert towards aprotic solvents, which have only little interaction with the latter, the liquid electrolyte fills the porosity of the electrode. Examples of batteries utilizing a liquid electrolytic component are found in U.S. Pat. Nos. 5,422,203; 5,626,985 and 5,630,993.
When the electrolytic component is in the form of a dry polymer matrix, it consists of a high molecular weight homo or copolymer, which is cross-linkable or non cross-linkable and includes a heteroatom in its repeating unit such as oxygen or nitrogen for example, in which an alkali metal salt is dissolved such as LiN(SO2CF3)2, LiSO3CF3 and LiCIO4. Polyethylene oxide is a good example of a polymer matrix which is capable of solving different alkali metal salts. Armand, in U.S. Pat. No. 4,303,748, describes families of polymers which may be used as electrolytic component in lithium batteries. More elaborated families of polymers (cross-linkable or non cross-linkable copolymers and terpolymers) are described in U.S. Pat. Nos. 4,578,326; 4,357,401; 4,579,793; 4,758,483 and in Canadian Patent No. 1,269,702. The use of a high molecular weight polymer enables to provide electrolytes in the form of thin films (of the order of 10 to 100 μm) which have sufficiently good mechanical properties to be used entirely as separator between the anode and the cathode while ensuring ionic transport between the electrodes. In the composite, the solid electrolyte serves as binder for the materials of the electrode and ensures ionic transport through the composite. The use of a cross-linkable polymer enables to utilize a polymer of lower molecular weight, which facilitates the preparation of the separator as well as the composite and also enables to increase the mechanical properties of the separator and, by the same token, to increase its resistance against the growth of dendrites when using a metallic lithium anode. As is well known in the art, repeated charge/discharge cycles can cause growth of dendrites on the lithium metal electrode. These dendrites can grow to such an extent that they penetrate the separator between positive and negative electrodes and create an internal short circuit. For this reason, metallic lithium anodes are used exclusively with solid polymer electrolyte separator sufficiently resistant and opaque to prevent dendrite growth from piercing its layer and reaching the positive electrode. Contrary to a liquid electrolyte, a solid polymer electrolyte is safer because it cannot spill nor be evaporated from the generator. Its disadvantage results from a lower ionic mobility obtained in these solid electrolytes which restricts their uses at temperatures between 40° C. and 100° C.
The gel electrolytic component is itself generally constituted of a polymer matrix which is solvating or non-solvating for lithium salts, aprotic solvent, and an alkali metal salt being impregnated in the polymer matrix. The most common salts are LiPF6, LiBF4, and LiN(SO2CF3)2 and the polar aprotic solvents may be selected from propylene carbonate, ethylene carbonate, butyrolactones, and 1,3-dioxolane, to name only a few. The gels may be obtained from a high molecular weight homo or copolymer which is cross-linkable or non cross-linkable or from a cross-linkable homo or copolymer. In the latter case, the dimensional stability of the gel is ensured by cross-linking the polymer matrix. Polyethers including cross-linkable functions such as alkyls, acrylates or methacrylates are good examples of polymers which may be used in formulating a gel electrolyte, such as described in U.S. Pat. No. 4,830,939. This is explained by their capacity to solvate lithium salts and their compatibility with polar aprotic solvents as well as their low cost, and ease of handling and cross-linking. A gel electrolyte has the advantage of being handled as a solid and of not spilling out of the generator as is the case with liquid electrolyte generators. Ionic transport efficiency is associated with the proportion of aprotic solvent incorporated in the polymer matrix. Depending on the nature of the polymer matrix, the salt, the plasticizing agent and its proportion in the matrix, a gel may reach an ionic conductivity of the order of 10−3 S/cm at 25° C. while remaining macroscopically solid. As in the case of a dry electrolyte, a gel electrolyte may be used as separator between the anode and cathode while ensuring ionic transport between the electrodes. In the composite electrode(s) of the generator, the gel electrolyte is used as binder for the materials of the electrode(s), and ensures ionic transport through the composite electrode(s). However, the loss of mechanical property resulting from the addition of the liquid phase (aprotic solvent) should generally be compensated by the addition of solid fillers, by cross-linking the polymer matrix whenever possible, or in some cases, when the proportion of liquid is too high, by using a porous mechanical separator which is impregnated with the gel which serves as electrolytic component in the separator.
The poor resistance of polyethers towards oxidation is however an important problem which is associated with the utilization of solid and gel electrolytes based on polyether as the electrolytic material in which the voltage in recharge may reach and even exceed 3.5V to 3.7 V. This results in an important loss of capacity of the generator which is caused by the more or less massive degradation of the polymer matrix during consecutive cycles of discharge/charge.
Gustafson et al. (U.S. Pat. No. 5,888,672) disclose a battery where the anode, the cathode, and the electrolyte each comprise a soluble, amorphous, thermoplastic polyimide. Since the polyimides are pre-imidized prior to the fabrication of the battery, there is no need to further cure them at high temperatures, thus reducing the risk of damaging the battery. The polyimide based electrolyte is resistant towards oxidation and capable of high ionic conductivity at or near room temperature. Nor is there a chance of incidental condensation as the battery temperature rises. In addition, since no further polymerization will occur, there are no by-products of the condensation reaction (water) to interact with the lithium salts. The battery of Gustafson et al. is said to be a dry cell.
In fabricating the battery, an electrolyte solution comprising a soluble, amorphous, thermoplastic polyimide solution and a lithium salt is prepared. The thermoplastic polyimide solution is prepared by mixing about 8% to about 20% by weight of a thermoplastic polyimide powder with about 80% to about 92% by weight of a solvent. About 20% to about 35% by weight of a lithium salt is dissolved in about 65% to about 80% by weight of a solvent to form a solution. The solution is then mixed with the thermoplastic polyimide solution to form the electrolyte solution. The electrolyte comprises from about 2% by weight to 10% by weight of soluble, amorphous, thermoplastic polyimide, from about 1% by weight to 12% by weight of the lithium salt and from about 78% by weight to 97% by weight of the solvent. An electrolyte layer is then formed by casting a film of the electrolyte solution which is fully dried in an oven at about 150° C. for about 30 to 60 minutes to create a dry, opaque, flexible, smooth, tough film. The polyimide based electrolyte solution is dried at the flash point of the solvent for the purpose of removing the solvent such that a dry electrolyte is obtained. The soluble, amorphous, thermoplastic polyimide may be any soluble, amorphous, thermoplastic polyimide known to those skilled in the art.
Laboratory tests have since demonstrated that a dried polyimide based electrolyte separator has poor ionic conductivity and as such is inadequate for battery applications with a metallic lithium anode. As described above, a lithium metal anode requires an electrolyte separator that presents enough mechanical resistance to prevent potential dendrite growths from piercing the electrolyte separator layer, reaching the positive electrode and causing a short circuit but it also requires a minimum of ionic conductivity to perform as an electrochemical generator.
Gustafson et al. (U.S. Pat. No. 6,451,480 issued Sep. 17, 2002) later on disclosed a Polyimide-based lithium-ion battery in which the anode and the cathode are prepared from an electrolyte polyimide binder solution comprising from about 9% by weight to about 15% by weight of a pre-imidized soluble, amorphous, thermoplastic polyimide powder dissolved in about 75% by weight to about 85% by weight of a polar solvent; and from about 6% by weight to about 12% by weight of a lithium salt and an electrolyte separator consisting of a typical separator film saturated with a liquid electrolyte solution of lithium salts dissolved in a variety of organic solvents such as ethylene carbonate mixed with dimethyl carbonate. A cell stack is assembled and placed in a container which is then filled with the electrolyte and sealed. Evidently, the ionic conduction is achieved by the solvent content of the electrolyte separator and of the composite anode and cathode. The cell is based on Li-ion technology using a liquid electrolyte and therefore cannot be combined with a metallic lithium anode as the high solvent content renders the electrolyte separator unable to prevent dendrites growth. Furthermore, the typical solvent used in Li-ion technology are unstable with metallic lithium anode; their resistance increasing with time.
Thus there is a need for an improved polyimide-based electrolyte having good ionic conductivity and capable of operating with a lithium metal anode.